[JITLink] Factor out forEachRelocation() function from addRelocations() in ELF...

[JITLink] Factor out forEachRelocation() function from addRelocations() in ELF Aarch64 backend (NFC)

First step in reducing redundancy in `addRelocations()` implementations across ELF JITLink backends. The patch factors out common logic for ELF relocation traversal into the new helper function `forEachRelocation()` in the `ELFLinkGraphBuilder` base class. For now, this is applied to the Aarch64 implementation. Others may follow soon.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D109516